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Petrea Wait, District Veterinarian, Local land Services, Monaro

Posted Flock & Herd February 2019
Part one can be found here

August 2018

So, now that you have been bombarded with all this new information on how to manage worms in your horses, you may be feeling more confused than ever. Let’s face it, giving a drench every six weeks was easy to remember, but now you have to consider a much larger amount of information before deciding on when, and if, to drench and with what.

What follows are some practical examples based on a real horse enterprise with a variety of horses of different ages and management types. The property is located in the South Tablelands of NSW, where temperatures can range from mid to high 30s in summer, with a cold winter that includes regular frosts and occasionally snow. There are several large paddocks with improved pasture, oat and lucerne crops are sown and used for grazing and hay making, and there are also a number of smaller quarter-acre paddocks that are used in rotation for foaling and young stock. In addition, there is 10 acres of unimproved native pasture on the less arable portion of the property. Horses are the only livestock run on the property, although cattle and sheep are sometimes "borrowed" from the neighbours as part of the pasture management program. Broodmares and race horses come and go from the property and foals are born and reared there. The horses are kept in small groups depending on age and nutritional requirements.

Tables 1 and 2 (below) show the results of worm egg counts carried out on manure samples collected in mid-December. Normally a worm egg count (WEC) would be conducted in spring, but in this particular year there was no rainfall until late October so testing was delayed until a time when pasture larval burdens would have increased after winter. From the results you can see that the horses last received a drench in winter of Equimec Triple, a product that contains Ivermectin, Oxibendazole and Praziquantel (effective against tapeworms).  

microscopic image of horse worm eggs

You will also notice that the types of worm eggs are divided into strongyle eggs and ascarid eggs. This differentiation is because ascarids have a large egg that is easy to identify, whilst the eggs of strongyles all look the same, be they large or small strongyle.  Table 3 shows the results of larval differentiation, a procedure where the strongyle eggs are hatched and then the number and type of strongyle larvae determined. In this case there were 100% small strongyles, a common finding in horse worm populations today, with large strongyles being highly susceptible and largely eradicated by the use of "mectin"-based wormers.

Table 1: Worm egg count results on manure samples collected in mid-December.

Faecal Egg Count - Horse

Lab. no. Sample ID Strongyle eggs Strongyle eggs interpretation Ascarid eggs Drench Drench date
0001 Betty 540 High 220 Equimec Triple 22 Aug 2017 14:20
0002 Sapphire 220 Moderate 1200 Equimec Triple 22 Aug 2017 14:20
0003 Skye 300 Moderate 0 Equimec Triple 22 Aug 2017 14:20
0004 Dutch 300 Moderate 0 Equimec Triple 22 Aug 2017 14:20
0005 Paris 80 Low 0 Equimec Triple 22 Aug 2017 14:20
0006 Jack 120 Low 0 Equimec Triple 22 Aug 2017 14:20
Table 2: Worm egg count results on manure samples collected in mid-December.

Faecal Egg Count - Horse

Lab. no. Sample ID Strongyle eggs Strongyle eggs interpretation Ascarid eggs Drench Drench date
0001 Coconut 0 Low 220 Equimec 22 Aug 2017 14:26
0002 Ernie 20 Low 660 Equimec 22 Aug 2017 14:26
0003 Sandy 0 Low 100 Equimec 22 Aug 2017 14:26
0004 Basil 0 Low 480 Equimec 22 Aug 2017 14:26
0005 Meg 740 High 340 Equimec 22 Aug 2017 14:26
0006 Blaze 0 Low 20 Equimec 22 Aug 2017 14:26
0007 Maisie 0 Low 60 Equimec 22 Aug 2017 14:26
0008 Duncan 580 High 20 Equimec 22 Aug 2017 14:26
0009 Shady 40 Low 0 Equimec 22 Aug 2017 14:26
0010 Ash 280 Moderate 0 Equimec 22 Aug 2017 14:26
Table 3: Larval differentiation on manure samples collected in mid-December.

Larval Differentiation for Horse Wormtest

Lab. no. Sample description Cyathostomes (non-migratory small strongyles)
P001 Samples 1-6 100%
Case 1 – Ernie
image of horse

Ernie is a two-year-old Thoroughbred colt. He was born on the property, but his mother visited a stallion on another stud before returning to foal down. The mare received a drench one month prior to foaling, and then Ernie was drenched at three months, six months and nine months in his first year of life. Ernie shares a small paddock with a four-year-old gelding, and they are moved to a new small paddock approximately every four weeks.

Ernie had a WEC of 20 strongyles and 660 ascarids. The strongyle count was under the 200 eggs per gram (epg) mark where we would consider drenching, but his ascarid counts was very high and as these are the worm type that cause the greatest problem in young horses a drench was definitely indicated. The drench selected was Strategy-T, a product that contains Oxfendazole and Pyrantel Embonate.   Oxfendazole was chosen for its good activity against ascarid worms, whilst Pyrantel has activity against small strongyles. As Ernie had received a "mectin" + Praziquantel-based drench in winter, tapeworms, bots and pin worms had been dealt with. Summer is also a good time to avoid a "mectin"-based wormer as bots are in their fly stage, and so not present in the horses’ gut in significant numbers, and therefore reducing the use of "mectin" drenches at this time of year reduces the risk of drench resistance in bots. 

Management practices that could be undertaken to reduce the likelihood of Ernie becoming reinfested with worms might include removing all manure from the paddocks once or twice weekly, providing all feed in bins and hay nets, and ensuring the paddock that he moves to next has been spelled for three months over summer or six months over winter.

Case 2 – Meg
image of horse

Meg is a 12-year-old Morgan X Riding Pony who is best described as a very good doer. Although she has never suffered from laminitis (founder), she is of a body type and breed that would increase her risk of this condition. As a result, she is also kept to a small paddock and tends to follow the young thoroughbreds after they have grazed most of the grass short. As she is always grazing close to the ground and following the 'wormy' young horses, she is exposed to a high level of worm larvae. This presumed exposure is supported by her WEC, with 740 epg strongyles and 340 epg ascarids. It is interesting to note that despite her high worm burden she never shows any symptoms of being 'wormy' (weight loss / poor condition, poor coat condition, scouring, or colic).

As Meg's strongyle count is over the 200 epg mark where we would consider drenching and her ascarid counts is high a drench is indicated, and the oxfendazole + pyrantel combination was selected for the same reasons as above for Ernie. 

Management practices that could be used to reduce Meg's worm burden could include removal of manure from the pastures weekly, grazing the paddock with cattle or sheep before it is grazed by Meg, or utilising the native pasture paddock as it provides a safer grazing option than the improved pastures for a founder-prone pony.

Case 3 – Jack
image of horse

Jack is a ten-year-old Thoroughbred gelding. He has been retired from racing and is now used for pleasure riding. He is kept at pasture with five other adult horses and intermittently has access to oat and lucerne crops. He tends to maintain body condition well and only receives a small hard feed after being exercised. His WEC showed 120 epg strongyles and no Ascarids. As the strongyle count is under 200 epg he did not receive a drench. Previous worm egg counts conducted on this horse over the last few years show that he rarely exceeds 200 epg, so he is likely to be one of the 80 percent of adult horses that maintain low worm burdens. He will receive a "mectin" and Praziquantal and BZ/Pyrantel drench as his routine autumn drench with follow-up WEC in spring, and possibly again in summer if conditions are warm and mild.

Case 4 – Coconut
image of horse

Coconut is a 22-year-old Thoroughbred mare that has been retired from breeding. She is kept at pasture and is used as a calm and sensible 'aunty' to the spelling race horses. She does not maintain her weight well and receives a hard feed and hay twice daily. Her WEC showed no worm eggs of either type so she did not receive a drench. It is possible that the manure for the test was collected at a time when she was not shedding eggs, so we cannot conclude that she is a low egg shedder from a single test, and a follow-up test is indicated in 6-8 weeks’ time. If she does continue to maintain a low WEC, a routine autumn drench will be her next worm treatment with another test conducted in spring.

Drench Check

Drench Check provides a simple, fast and low-cost indication of possible drench resistance. It involves the use of two WECs, one used before and one after a drench, to see how much the drench reduces the WEC. The first WEC within the Drench Check is done up to ten days before the horse is drenched, and the second is done exactly 14 days after drenching for most drenches, or 42 days later for a long-acting drench that contains Moxidectin as one of the active ingredients. A drench is considered effective if it reduces the WEC by 95% or more. Below in Table 4 are the results of a Drench Check conducted 14 days after the above horses received their drench.

Table 4: Results of a Drench Check conducted 14 days after drenching. 
0001 0002 0003 0004 0005 0006
Meg Ernie Ash Basy Evie Betty
Results Faeces 1 Faeces 2 Faeces 3 Faeces 4 Faeces 5 Faeces 6
Strongyle eggs 0 0 0 0 40 20
Strongyle eggs interpretation Low Low Low Low Low Low
Ascarid eggs 0 0 0 0 0 0
Drench Strategy T Strategy T Strategy T Strategy T Strategy T Strategy T
Drench date 7 Jan 2018 17:09 7 Jan 2018 17:09 7 Jan 2018 17:09 7 Jan 2018 17:09 7 Jan 2018 17:09 7 Jan 2018 17:09

In the cases of Ernie and Meg, the egg count was reduced to zero eggs per gram for both worm types, indicating the drench was highly effective. In the case of Betty, her initial Strongyle count was 540 epg (Table 1), which reduced to 20epg after the drench (Table 4). To calculate the effectiveness of the drench the following formula is used:

(WEC1 - WEC2) ÷ WEC1 x 100 = % effectiveness


(540 - 20) epg ÷ 540 epg x 100 = 520 ÷ 540 x 100 = 96%

Therefore the drench was 96% effective in reducing the worm burden, and thus was a good choice. It also indicates that if this drench is used in the future it should also be effective as the worms present on the property have very little resistance to the active ingredients. This result does not mean that the drench should be used repeatedly but can still be used as part of the drench rotation program. A drench with at least one different ingredient should be used as the next drench given.

The case studies above highlight the need to develop worm control strategies that are customisable to meet the specific requirements of different properties and horses depending on factors such as horse age and breed, weather condition and pasture management. The case studies also highlight that environmental management should be considered as an important part of any worm control strategy. 


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